This invention relates in general to heat exchange arrangements. In particular, the invention relates to a heat exchange arrangement which is essentially composed of metallic material and serves to provide heat exchange for chemically aggressive fluids, such as sulfuric acid and the like.
Heat exchangers per se are not novel, and it is well known that many of them include conduits through which the fluid which is to undergo a temperature change is made to flow. These conduits are then contacted by a heat exchange medium, for instance a gaseous cooling medium or the like, and the outer surfaces which are so contacted may be either smooth or they may be provided with fins to facilitate the heat transfer.
Heat exchange arrangements of this general type are generally very effective and quite versatile. However, under certain circumstances, metallic heat exchange arrangements of this type present special problems. This is true, for instance, where such heat exchange arrangements are used for cooling of chemically aggressive fluids, especially of sulfuric acid, because these fluids tend to relatively rapidly corrode the material of the heat exchanger. As a general rule it has been found that the average temperature of the walls of the heat exchanger will play a significant role in determining the degree of corrosion susceptibility of the heat exchanger material. Usually, the corrosion susceptibility of the heat exchanger material increases as the temperature increases. When water is used as a cooling medium, the average wall temperature is lower than in cases where air is used as the cooling medium; this is the result of the fact that the heat transfer between water and metal is considerably better than that between air and metal. Since the heat transfer between air and metal is poorer than that between water and metal, the average wall temperature in the former case is necessarily higher than in the latter case and, in consequence, the air-cooled heat exchangers which are known from the prior art for cooling aggressive fluids are particularly susceptible to the corrosive effects of the fluid.
Ever since the aforementioned problem was identified, it has been attempted in the industry--where aggressive fluids are to be cooled by heat exchange with air--to maintain the average wall temperature as low as possible or, to put it another way, to have the temperature of the fluid to be cooled as low as possible before it is introduced into the heat exchanger. For example, where air-cooling of sulfuric acid is involved, the maximum permissible temperature prior to introduction of the acid into the heat exchanger will lie between 80.degree. and 85.degree.C. This temperature limit applies to a highly concentrated solution, where the sulfuric acid concentration is between 96 and 98% by weight. However, this temperature limit can be obtained only where the heat exchanger is composed of expensive, highly alloyed steel, such as, for instance, X 10 Cr Ni Ti 18 9 or X 10 Cr Ni Mo Ti 18 10. Even then, corrosive effects will become apparent within a relatively short period of time.
It is also known in the art that if the flow rate of the aggressive fluid through the heat exchanger is high, a better heat transfer will be obtained. It was found, however, particularly with reference to sulfuric acid, that the corrosive action of the acid upon the metal of the heat exchanger increases significantly when a flow rate of 1 meter per second is exceeded.
All of the limitations imposed upon the prior art by the aforementioned considerations are quite disadvantageeous from a point of view of economy of operation. It is well known that it is particularly favorable to set up and operate sulfuric acid installations in such a manner that the protection temperature of the sulfuric acid can exceed 85.degree.C. It will be recalled that, as mentioned above, heretofore this has been the critical temperature limit that could not be exceeded. To be able to operate at a temperature of the sulfuric acid in excess of 85.degree.C means, however, that for instance in the contact process for making sulfuric acid, the process may be carried out more economically and with less of a capital inventment when higher temperatures can be employed in the contact towers. Moreover, when the temperature at which an aggressive fluid, and in particular sulfuric acid, is introduced into a heat exchanger can be increased beyond the 85.degree.C limit, the specific thermal efficiency of the heat exchanger is also increased, due to the greater temperature differential which is obtained. It is thus clearly desirable to inhibit the corrosive effects of chemically aggressive fluids in heat exchangers, and to be able to operate such heat exchangers with the aggressive fluids, especially the sulfuric acid, at temperatures in excess of 85.degree.C.